1. Field of the Invention
The present invention relates generally to flat panel displays. More specifically, the invention relates to a thin-film transistor (TFT) of a gate driving module.
2. Description of the Related Art
The Liquid Crystal Display (LCD) is a flat panel display that includes an LCD panel. Typically, an LCD panel has a lower substrate on which pixel electrodes may be formed, an upper substrate on which a common electrode may be formed, a liquid crystal layer interposed between the lower substrate and the upper substrate, and a driving module which drives the LCD panel. An LCD displays an image by altering an electric field between the common electrode and the pixel electrodes in the LCD panel to vary the optical transmittance of the liquid crystal layer.
An increasing number of LCDs are integrating gate driving modules onto one or both of the substrates in order to reduce manufacturing cost. That is, thin-film transistors (TFTs) and pixel electrodes may be formed on a display region of the lower substrate, and a gate driving module may be formed on a peripheral region. The gate driving module includes two or more stages and is thus able to apply a gate-on voltage or a gate-off voltage to gate lines in response to a clock signal, an inverted clock signal, or a frame initiation signal, which is a type of control signal. As a result, the TFTs are sequentially turned on for each frame. Each stage of the gate driving module can include an amorphous silicon gate (ASG) circuit which includes ASG TFTs. An ASG circuit performs the same or similar functions as those of a gate driver integrated circuit (IC).
The TFTs of the gate driving module serve as switches that apply a gate-on voltage or a gate-off voltage to their respective gate lines in response to the clock signal, the inverted clock signal, or the frame initiation signal. The TFTs of the gate driving module can be formed when the TFTs in the display region are formed.
In general, TFTs are classified into I-type TFTs which have a source electrode that is formed as a straight line, or U-type TFTs which have a source electrode that is U-shaped. Typically, the TFTs in the display region are all U-type TFTs, whereas some of the TFTs of the gate driving module are U-type TFTs and the other TFTs of the gate driving module are I-type TFTs. Here, the I-type TFTs commonly maintain the ratios of the capacitance between a gate electrode and a drain electrode, and the capacitance between the gate electrode and a source electrode, at 1:1 and provide a channel length long enough to secure sufficient resistance.
However, the active layer of a U-type TFT can be more easily removed than the active layer of an I-type TFT due to the difference between the thicknesses of their respective photosensitive layers. The thickness of the active layers of the TFTs in the display region may differ from the thickness of the active layers of the TFTs in the gate driving module by as much as 1000-2000 Å (Angstroms). Due to this difference in thicknesses, the active layers of some of the TFTs of the gate driving module, where a long channel length is secured by using a half-tone mask, may be inadvertently removed during etching. Thus, the active layers of the TFTs of the gate driving module may not form stable channels, thereby causing the gate driving module to malfunction.